In recent years, carbon materials have been widely used in the electricity storage field. Among such electricity storage carbon materials, activated carbons, which have a relatively large surface area are used as electrodes of electrical double layer capacitors while coke, which has a relatively small surface area is used as negative electrodes of lithium ion secondary batteries. In particular, electric double layer capacitors have drawn attention because they can be used as backup power supplies, auxiliary power supplies or the like, and in this connection, various development focusing on the properties of activated carbon as electrodes of electric double layer capacitors have been widely carried out. An electric double layer capacitor wherein an activated carbon is used as polarizable electrodes is excellent in electrostatic capacitance and thus the demand for the capacitor as electrodes for various electric devices has been rapidly growing, accompanying with development in the field of electronics. Further, recently in addition to downsizing of conventional memory backup power supplies, various developments of large capacity products to be used as an auxiliary power supply for motors have been carried out.
These activated carbons are generally produced by subjecting carbonaceous materials to activation using gas or chemicals, for example, alkaline activation using an alkali metal hydroxide as an activation agent and then neutralizing the activated materials by washing with a strong acid such as hydrochloric acid, nitric acid, or sulfuric acid to remove alkali metals and heavy metals.
In connection with a demand for high performance electric double layer capacitors, there have arisen needs to further decrease the alkali metals remaining in the activated carbon used as the electrodes of the capacitor. However, as described in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2005-123462), the alkali metal can be removed to a certain concentration by repeatedly washing with acid or water, but further removal of the metal is difficult. Therefore, an electric double layer capacitor with a carbon material having been subjected to alkaline activation is excellent in initial capacitor capacity but has a defect concerning deterioration over time that it is large in loss of capacitor capacity after the long time use.
On the other hand, in view of washing treatment, it is desirously as simple as possible, i.e., the number of times of washing is desirously as fewer as possible to lower the cost. Patent Document 2 (International Publication No. WO2004/011371) describes that activated carbon is produced by subjecting an easily graphitizable carbonaceous material to an alkali activation treatment and then washing the resulting activated product with hot water, carbonated water, hot hydrochloric acid, aqueous ammonia, hot hydrochloric acid and hot water in this order. However, this washing treatment is tedious and thus there has arisen a strong demand of development of activated carbon with a structure enabling remaining alkali metals to be readily removed.
Needle coke is used as aggregate for graphite electrodes used in electric furnace steel making processes and is generally produced using petroleum-based heavy oil or coal tar as the raw material. In a process of producing a graphite electrode, coke particles and a binder pitch are blended at a predetermined ratio, and then kneaded while being heated, and extrusion-molded thereby producing a green electrode. The green electrode is calcined to be graphitized and fabricated thereby producing a graphite electrode product.
The graphite electrode is desirously lower in coefficient of thermal expansion (CTF) because it is used under severe conditions such as high temperature conditions. That is, a graphite electrode with a lower CTF is less consumed and thus can reduce the cost of the electric furnace steel making.
The above-mentioned graphitization is a process wherein a green electrode is heated at a temperature of about 3000° C. and in general a direct current flow furnace (LWG furnace) is used. However, graphitization carried out in the LWG furnace accelerates a temperature elevating rate and thus increases a generating rate of gas. As the result, an abnormal expansion phenomenon, so-called puffing is likely to occur. Puffing lowers the density of an electrode and also sometimes breaks the electrode.
Now, a method has been studied wherein coefficient of thermal expansion and puffing are controlled upon production of needle coke, and there have been proposed various methods. For example, Patent Document 3 (Japanese Patent Laid-Open Publication No. 5-105881) discloses a method wherein a coal tar pitch from which quinoline-insolubles has been substantially removed is blended with an oligomer adjusted in polymerization degree and coked by the delayed coking method. Patent Document 4 (Japanese Patent Laid-Open Publication No. 5-163491) discloses a method wherein a coal tar-based heavy oil and a petroleum-based heavy oil are blended at a specific ratio such that the nitrogen and sulfur contents are to be 1.0 percent by mass or less and 1.4 percent by mass or less, respectively to prepare a feedstock which is then placed into a delayed coker to produce a green coke, which is then calcined at a temperature of 700 to 900° C. and cooled, and again calcined at a temperature of 1200 to 1600° C. Patent Document 5 (Japanese Patent Laid-Open Publication No. 5-202362) discloses a method wherein upon production of coal tar by rapid thermal cracking of coal, the thermal cracking temperature in the reactor is kept at 750° C. or higher and the residence time of the thermal cracked product in the reactor is 5 seconds or shorter thereby producing a liquid product which or the pitch of which is then carbonized. Patent Document 6 (Japanese Patent Laid-Open Publication No. 7-3267) discloses a method wherein needle coke is produced by subjecting a petroleum-based heavy oil alone or a mixture thereof with a coal tar-based heavy oil from which quinoline-insolubles have been removed, as the feedstock to delayed coking and thereupon the petroleum-based heavy oil has been so adjusted that the content of particles such as ash therein is to be from 0.05 to 1 percent by mass.
However, any of the methods described in Patent Documents 3 to 6 is not necessarily sufficient in lowering coefficient of thermal expansion or inhibition of puffing and it is actual situation that the quality of the coke produced by these methods has not reached to the level required for aggregate of graphite electrodes used in electric furnace steel making processes.